This application claims the priority benefit under 35 U.S.C. § 119 of Japanese Patent Application No. 2005-334311 filed on Nov. 18, 2005, which is hereby incorporated in its entirety by reference.
1. Field
The disclosed subject matter relates to a white LED illumination device, and in particular, relates to a white LED illumination device having a white LED as a light source. The white LED has an LED chip and a wavelength conversion material. The LED chip can emit light having a peak wavelength in, for example, the blue wavelength range. The wavelength conversion material such as a fluorescent material is excited by the light from the LED chip and performs wavelength conversion to emit, for example, yellow or yellowish green fluorescence which is a complementary color of blue.
2. Brief Description of the Related Art
LED chips generally emit light that has steep spectral characteristics (spectral distribution). Humans typically recognize the light as light with a tone approximately corresponding to its peak wavelength λp (wavelength at which maximum emission intensity is achieved). Accordingly, the LED chip emits light in an intrinsic tone caused by the material, composition, structure, and the like of the LED chip. This emitted light is generally not white light (natural light) with superior color rendering properties like that produced by the sun (which includes a wavelength component in a wide wavelength range throughout the ultraviolet, visible, and infrared regions).
There are some methods to obtain white light by using the LED chip emitting light with such spectral characteristics as a light source. In one method, three kinds of LED chips, that is, an LED chip emitting red light (red LED chip), an LED chip emitting green light (green LED chip), and an LED chip emitting blue light (blue LED chip) are employed. They are turned on at the same time, and as a result, white (W) light with a desired tone can be generated by means of additive color mixture. The tone can be adjusted by independently controlling the respective amounts of red (R) light, green (G) light, and blue (B) light which are the three primary colors and are emitted by the respective LED chips.
This method has a disadvantage in that three driving circuits are necessary to independently control the respective amounts of light emitted by the LED chips. However, there is also an advantage in that the tone of light generated by means of the additive color mixture can be successively controlled.
Another method uses a fluorescent material serving as a wavelength conversion material. This method utilizes a principle in which when a fluorescent material is irradiated with light, the fluorescent material is excited and emits light with a longer wavelength than that of the excitation light.
Specifically, for example, the fluorescent material such as YAG, TAG, or orthosilicate is available. When blue light (light having a peak wavelength in the blue wavelength range) is emitted from an LED chip made of a semiconductor material such as ZnSe, InGaN, GaN, ZnO, etc., excites the fluorescent material, the fluorescent material emits, via wavelength conversion, yellow and/or yellowish green fluorescence which are complementary colors of the blue light. Yellow light, which is generated by wavelength conversion when part of the blue light emitted from the LED chip excites the fluorescent material, can be added to or mixed with part of the blue light emitted from the LED chip to generate white light by means of the additive color mixture.
In another example, when an LED chip emits blue light, a mixture of two kinds of fluorescent materials can be employed, which emit green and red fluorescence, respectively, by wavelength conversion when excited by the blue light. Green light and red light, which are generated by wavelength conversion when part of the blue light emitted from the LED chip excites the fluorescent materials can be mixed with or added to part of the blue light emitted from the LED chip to generate white light by means of additive color mixture.
When an LED chip emits ultraviolet light, a mixture of three kinds of fluorescent materials can be employed, which emit blue, green, and red fluorescence, respectively, by wavelength conversion when excited by the ultraviolet light. Blue light, green light, and red light, which are generated by wavelength conversion when part of the ultraviolet light emitted from the LED chip excites the fluorescent materials, can be mixed together and generate white light by means of additive color mixture.
Furthermore, when the wavelength of light emitted from an LED chip and a kind of fluorescent material are appropriately selected and combined, light in various tones other than white light can also be generated.
FIG. 1 is an example of such an LED. In this LED, part of the light emitted from a light source excites a fluorescent material for wavelength conversion and generates light different in tone from the light emitted from the light source. The LED has a printed circuit board 53 in which conductive patterns 52a and 52b are formed on an insulating substrate 51. A lamp house 56 is attached to the printed circuit board 53. The lamp house 56 is provided with a bowl-shaped recessed portion 55 having a reflective surface 54 opening upward and outward. A blue LED chip 58, which can emit blue light, is mounted on the conductive pattern 52a formed on an inner bottom 57 of the recessed portion 55 (on the printed circuit board 53) via a conductive adhesive 59 to electrically connect a lower side electrode of the blue LED chip 58 to the conductive pattern 52a. An upper side electrode of the blue LED chip 58 is electrically connected to the conductive pattern 52b through an overhead wired bonding wire 60. Furthermore, a transparent resin 62, into which a fluorescent material 61 is mixed, is filled into the recessed portion 55. The blue LED chip 58 and the bonding wire 60 are sealed by the resin to be shielded from air. In this instance, when the blue light excites the fluorescent material 61, the fluorescent material 61 can emit via wavelength conversion yellow or yellowish green fluorescence, which are complementary colors of blue.
In the LED 50 with such a structure, the light emitted from the blue LED chip 58 reaches a light emitting surface 63 through the transparent resin 62 into which the fluorescent material 61 is mixed. In this case, the optical path and optical path length of the light differ according to an emission direction of the light from the blue LED chip 58. To be more specific, when L1 represents light emitted from the blue LED chip 58 in an optical axis X direction (upward) of the blue LED chip 58 toward the light emitting surface 63, d1 represents its optical path length. L2 represents light emitted from the blue LED chip 58 in a slanting upward direction from the blue LED chip 58 toward the light emitting surface 63, and d2 represents its optical path length. L2 is longer than L1, and therefore, the difference of the optical paths is represented by d2-d1.
Such difference in the optical path means that there is difference in a ratio of wavelength conversion of the light from the blue LED chip 58 by the fluorescent material. In other words, the light L1 reaching the light emitting surface 63 through the short optical path has a blue tone because it is subjected to a low ratio of wavelength conversion by the fluorescent material. On the contrary, the light L2 reaching the light emitting surface 63 through the long optical path has a yellow tone because it is subjected to a high ratio of wavelength conversion by the fluorescent material.
Accordingly, this LED emits bluish white light in a front direction and emits yellowish white light in the slanting upward direction. Also, the bluish white light is emitted from the center of the light emitting surface of the LED and the yellowish white light is emitted from the periphery of the light emitting surface of the LED. Thus, the LED has an optical characteristic with an uneven tone because the emitted white light has different tones according to its emission direction and emission portion.
By the way, it is conventionally known that a light source that evenly emits light in every direction, like the above-described LED, is optically regarded as a point source of light. This is true when the size of the light source is much smaller than the distance from which it is observed (observation distance is approximately five times or more of the size of the light source).
Consider that an illumination device for illuminating a position that is approximately five times or more of the size of the light source away is configured using the above-described LED as a light source. In this case, it is necessary to secure the illuminance of an illumination surface and improve unevenness in the tone of the light source by applying certain light-gathering means to the wide directional light emitted from the light source.
To cope with this, as shown in FIG. 2, there is an LED in which a condenser lens is provided in front of the LED in a light emission direction to collect the light to an illumination surface and even the tone of illumination light in the illumination surface.
Bluish white light emitted in a front direction of an LED 80 is refracted by an upper incident surface 82 of a lens 81 and introduced into the lens 81 while collected. The bluish white light reaches a center emitting surface 83 after passing through the lens 81, and is refracted by and gathered at the center emitting surface 83, and is emitted to the outside of the lens 81. Yellowish white light emitted in a side direction of the LED 80, on the other hand, is refracted by a side incident surface 84 of the lens 81 and introduced into the lens 81. The yellowish white light reaches a peripheral reflective surface 85 after passing through the lens 81. The light reflected (totally reflected) by the peripheral reflective surface 85 reaches a peripheral emitting surface 86, provided on the periphery of the center emitting surface 83, after passing through the lens 81. Then, the yellowish white light is emitted to the outside of the lens 81 from the peripheral emitting surface 86. In this instance, the yellowish white light emitted in the side direction of the LED 80 and emitted to the outside from the peripheral emitting surface 86 is gathered at the center of an illumination region, and is mixed with the bluish white light, which is emitted in the front direction of the LED 80 and emitted to the outside from the center emitting surface 83. Therefore, it is possible to obtain white light in even tone (for example, please see Japanese Patent Laid-Open Publication No. 2005-216782 and U.S. Patent Publication No. 2005-179064, which are hereby incorporated in their entirety by reference).
The white light in the different tones emitted in the different directions of the LED 80 are mixed on the illumination surface and the unevenness in tone is resolved so that it is possible to obtain even white light. The white light in the different tones emitted from the different sections of the LED 80, however, are incident on the illumination surface as-is. In addition to this, they are magnified through an optical system in which the light emitting surface of the LED 80 is composed of the lens 81, so that the unevenness in tone is not resolved. As a result, white light with unevenness in tone is still emitted through the illumination surface.